A ceramic honeycomb composed mainly of cordierite has hitherto been used extensively as a carrier for a catalyst for rendering an automobile exhaust pollution-free. In recent years, the advantages of a metallic honeycomb comprising a stainless steel foil have been recognized, and the metallic honeycomb has begun to be mounted on some high grade passenger cars, which has led to an increasing tendency that the number of the metallic honeycombs installed in automobiles has gradually increased. The advantages of the metallic honeycomb include the following: 1 since the temperature rise rate at the start of an engine is higher than that in the case of the ceramic honeycomb, the metallic honeycomb can begin to exhibit catalytic activity for purifying the exhaust gas earlier than the ceramic honeycomb, which contributes to a reduction in the amount of release of toxic gases; 2 since the wall thickness is half or smaller than that of the ceramic honeycomb, the exhaustion resistance becomes so small that the output loss of the engine is small; 3 the surface area per unit volume of the honeycomb is so large that it is possible to relatively reduce the size; and 4 in the ceramic honeycomb, a cushioning material comprising a wire of inconel or a high grade stainless steel should be interposed between the honeycomb and the jacket and, due to poor heat resistance of the cushioning material, there is a limitation on an increase in the temperature of the exhaust gas, whereas the metallic honeycomb is directly joined to the jacket, so that no cushioning material is needed and it can be disposed at a higher temperature portion immediately behind the engine manifold to shorten the build up time of the purification activity of the catalyst.
The metallic honeycomb can be used at a higher temperature than the ceramic honeycomb. In recent years, however, there is an ever-increasing demand for lean burn and high speed running with low fuel consumption against the background of tightening of CAFE and regulation of exhaust gases, which has lead to a tendency of a further increase in the temperature of the exhaust gas from an automobile engine, so that, in some cases, the conventional metallic honeycombs cannot satisfy the new heat resistance requirements.
Specifically, the inlet gas temperature to which the conventional metallic honeycombs have been exposed was about 850.degree. C. at the highest even when they were used immediately behind the engine manifold. In recent years, however, the inlet gas temperature often reaches 900.degree. to 1,000.degree. C. This has made it impossible for the conventional metallic honeycombs to pass a strict test for durability requirements.
For example, as described on pages 70 to 80 of "Nikkei Mekanikaru (Nikkei Mechanical)", No. 20 issued on January in 1992, an improvement in the oxidation resistance of the honeycomb has been considered as the first requirement to be satisfied for improving the heat resistance of the metallic honeycomb. For this reason, as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 92286/1975, 48473/1976 and 71898/1982, attention has been directed to the oxidation resistance of a foil material for a metallic honeycomb and the adhesion of a film, and use has been made of a foil comprising, as a base material, an Fe-Cr-Al-base alloy, which has hitherto been extensively used as a heating wire and a high-temperature member for heaters by virtue of its excellent oxidation resistance and adhesion as a film, and having improved heat resistance or adhesion to an activated alumina (y-Al.sub.2 O.sub.3) coated layer serving directly as a carrier for the catalyst. In all the techniques disclosed in the above-described publications, Y is utilized as means for improving the oxidation resistance of the material.
On the other hand, Japanese Examined Patent Publication (Kokoku) No. 2-58340 proposes an Fe-Cr-Al-base alloy containing 0.06% by weight in total of rare earth elements including 0.002 to 0.05% by weight of a rare earth element selected from La, Ce, Pr and Nd added mainly for the purpose of preventing peeling of an oxide film of the alloy and an Fe-Cr-Al-base alloy containing Zr for the purpose of stabilizing the alloy and Nb in an amount enough to satisfy a particular relationship thereof with the C content and N content for the purpose of ensuring the high-temperature creep strength. In these publications, there is a description to the effect that, when the total content of the rare earth elements exceeds 0.06% by weight, no significant improvement in the oxidation resistance can be attained as compared with a total content of 0.06% by weight or less and, further, it becomes impossible to effect working at conventional hot working temperatures.
Similarly, Japanese Unexamined Patent Publication (Kokai) No. 63-45351 proposes an Fe-Cr-Al-base alloy wherein the above-described rare earth element exclusive of Ce or La alone are added in an amount in the range of from 0.05 to 0.2% by weight because the addition of Y increases the production cost. According to the description of the publication, the proposal is based on a finding that the presence of Ce is causative of a lowering in the hot workability attributable to the addition of the above-described rare earth elements and also deteriorates the oxidation resistance and the addition of the rare earth elements exclusive of Ce alone renders the material hot-workable and further contributes to an improvement in the oxidation resistance. However, the rare earth elements are very reactive and similar to one another in the chemical properties, so that the separation of individual rare earth elements is not easy, which renders the separated rare earth elements very expensive as compared with a mischmetal comprising a conventional mixture of rare earth elements. For this reason, as with the sole use of La, the separation and removal of Ce alone inevitably incurs an increase in the cost. Further, Japanese Unexamined Patent Publication (Kokai) No. 63-42356 of which the applicant is the same as that of the above-described Japanese Unexamined Patent Publication (Kokai) No. 63-45351 discloses an Fe-Cr-Al-base alloy which is excellent in oxidation resistance and peeling resistance of oxide scale and contains 0.01 to 0.30% in total of Ce, La, Pr and Nd. In this publication, however, no study has been made on the hot workability of the alloy.
In the above-described prior art, although the adhesion and oxidation resistance of an oxide film have been studied with a view to improving the heat resistance of the metallic carrier, the influence of the foil material on the durability of the honeycomb structure, for example, high-temperature proof stress, which is a very important necessary property for a foil constituting a honeycomb of the catalyst from the practical point of view, has not been studied at all.
The present invention has been made with a view to solving the problem of lack of heat resistance of the conventional metallic carrier associated with an increase in the temperature of automobile engine exhaust, and an object of the present invention is to provide a highly heat resistant metallic carrier capable of withstanding engine exhaust having a higher temperature (900.degree. to 1,000.degree. C.).